Completeness (as a percentage) of symmetry-unique
               anomalous difference data within the sphere defined by
               the diffraction limits (_reflns.d_resolution_high and
               _reflns.d_resolution_low) relative to all possible
               symmetry-unique anomalous difference data within that
               sphere.
               In the absence of an anisotropy description this is
               identical to _reflns.pdbx_percent_possible_anomalous.